Method for making a ferromagnetic memory core



United States Patent 3,293,184 METHOD FOR MAKING A FERROMAGNETIC MEMORY CORE Gerrit Antonie Herman van Driel, Cornelis Jacobus Esveldt, and Petrus Johannes Flinsenherg, all of Emmasingel, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Dec. 6, 1963, Ser. No. 328,493 Claims priority, application Netherlands, Dec. 13, 1962, 286,715 1 Claim. (Cl. 252-625) Our invention relates to an annular magnet core suitable for use as a so-called storage element in a magnetic memory, and to a method of manufacturing such a magnet core.

The utility of a storage element in a magnetic memory is determined by its pulse characteristics or dynamic characteristics. In this respect, a distinct difference between the maximum value uVl of the undisturbed l-signal and the maximum value dVz of the disturbed 0-signal is important (for a good storage element the value uV1 and the value rVl, i.e. the maximum value of the disturbed l-signal, differ only slightly from each other).

It is a principal object of our invention to provide a magnet core meeting the requirement: uVl/dVz 3 with a disturbance ratio of 0.61. (For a definition of the terms uVl, rVl and (W2, see US. Patent 2,988,508.)

A further object of our invention is to provide a magnetic core as a storage element having a very short switching time.

A still further object of our invention is to provide a magnet core as a storage element which has a switching time not exceeding 0.25 ,usec.

In accordance with the invention, we have found that an annular core having an outer diameter not exceeding 0.6 mm. and an inner diameter which is at least half that of the outer diameter and which is constituted of a fired reaction product of lithium oxide, nickel oxide, and ferric oxide in the proportions of about 14 to 15 mol percent of Li O, about to 7 mol percent of NiO and about 78 to 81 mol percent of Fe O has a switching time not exceeding 0.25 ,usec. and a value of the quotient uVl/a'Vz 3 with a disturbance ratio of 0.61.

The magnet cores, according to the invention, are obtained by sintering a presintered product of a mixture of oxides of lithium, nickel and iron, which oxides may be replaced individually and Wholly or partly by an equivalent quantity of one or more other compounds of the metal concerned, which can be converted during the sintering process into the respective oxides, and in which mixture the relative quantities of lithium (calculated on Li O), nickel (calculated on NiO) and iron (calculated on Fe O are:

14 to 15 mol percent of Li O 5 to 7 mol percent of NiO, and 78 to 81 mol percent of F8203.

The best results are obtained by processing the aforesaid presintered porduct with a solution of polyvinyl alcohol in water to form a slurry, which is inspissated to a mass having aweight of 1.1 to 1.2 times that of the quantity of presintered product, after which the particles having a dimension exceeding of the outer diameter of the product to be pressed are sieved out, the smaller particles being dried and compressed to a product of the desired shape, having a density of 2.3 to 3.0 gms./cm. The compressed product is then heated within a period of 1 minute in air or in an air-oxygen mixture on a substratum of a high-melting-point metal or a high-meltingpoint metal alloy to a temperature lying between 1060 C. and 1180 C., which temperature is maintained for 1.5 to 10 minutes. The sintered product then is cooled at a rate of at least 5 C. per minute and at the most 20 C. per minute to a temperature which is at least C. below the sintering temperature, the product being finally quenched by contact with the air at room temperature.

Although the cores, according to the invention, have a rather high coercive force-at least 6 oerstedthis does not appear to be detrimental since they have small dimensions.

The invention will be described with reference to the following illustrative example, which not only specifies the switching time of the magnet cores, but also the peaking time. The term peak time (T of a magnet core denotes herein the period of time lying between the instant when the control-current attains an intensity of 10% of its maximum value and the instant when the output voltage of the l-signal, produced by the control-current pulse concerned is at a maximum. The peaking time is, of course, dependent upon the rising time (T,) of the control-current pulse. With the experiments relating to the present invention, the said rising time was always 0.05 sec.

Example A finely divided mixture of 14.52 mol percent of lithium carbonate Li CO 6.45 mol percent of NiO and 79.03 mol percent of iron oxide Fe O was presintered for 10 hours at a temperature of 625 C. and then cooled. The presintered product was ground to powder and 50 gms. of the powdery presintered product was mixed in a mortar with 25 cms. of a 6% solution of polyvinyl alcohol in water to obtain a Slurry. This slurry was inspissated at C. to a weight of 55 to 59 gms. Then the mass was sieved through ditferent sieves having a gradually smaller width of mesh, i.e. in order of succession: 600, 300, 100, 75, 60, 50, 42a. The fractions 60 to 50 6042p., or 5042,u. were dried for 16 hours and sieved again. These fractions were compressed in a die having tungsten carbide stampers to annular products having a density of 2.3 to 3.0 gms./cm. These compressed products were then sintered in air by heating on a substratum of platinum or a platinum-rhodium alloy in an electric furnace Within 45 sec. at a temperature of 1160 C., which temperature was maintained for 2 minutes. Subsequently, the sintered cores were cooled within a period of time of about 20 minutes within the furnace to a temperature of 1040 C. Finally, the sintered products were removed from the furnace and quenched by contact with air of room temperature.

The outer diameter of the sintered bodies obtained was 0.540 mm., the inner diameter 0.355 mm. The pulse characteristics, measured with a control-current of 995 ma., a pulse duration of 0.450 ,usec. and a rising time of 0.050 ,usec, were as follows:

uV1-=48 mv., rV1=44.6 mv., dVz=5.5 mv., T =0.190 ,usec., T =0.118 ,usec.

By varying the sintering temperature and the cooling temperature, i.e. instead of sintering at a temperature of 1160 C., sintering was at a temperature of 1080 C., and if the cooling temperature was 980 C. instead of 1040 C., annular cores having an outer diameter of 0.550 mm. and an inner diameter of 0.360 mm. were obtained. The pulse characteristics, measured with a control-current of 1485 3 ma., a pulse duration of 0.350 used, and a rising time of 0.050 ,usec. were:

uV1=31.4 mv., rV1-=28 mv., dVz=8.9 mv., T =0.153 1sec, T =0.102 p.866.

While we have described our invention with reference to specific examples and applications, other modifications thereof will be apparent to those skilled in this art without departing from the spirit and scope of the invention which is defined in the appended claim.

What we claim is: v

A method of manufacturing magnet core having an outer diameter not exceeding 0.6 mm. :and an inner diameter which is at least half that of the outer diameter, a switching time not exceeding 0.25 ,usec. and a value of the quotient uV1/dVz 3 with a disturbance ratio of 0.61 comprising the steps, heating at about 625 C. a finelydivided mixture of about 14 to 15 mol percent of Li O, about 5 to 7 mol percent of NiO, and about 78 to 81 mol percent of Fe O to form a presintered material, finely dividing and forming said presintered material into a slurry, increasing the consistency of the slurry to a mass having a weight 1.1 to 1.2 times that of the initial slurry,

removing particles from the thickened slurry which have a dimension exceeding A the outer diameter of the core, drying the slurry containing only smaller particles, compressing the latter pa-rticles into a shape of the core with a density of about 2.3 to 3.0 gms./cm. heating said core in an atmosphere containing at least as much oxygen as air on a substratum of a high-melting point metal in less than one minute up to a temperature between about 1060 C. and 1180 'C., maintaining the core at said temperature for about 1.5 to 10 minutes to sinter the same, cooling the sintered core at a rate of at least 5 C. per minute and not exceeding 20 C. per minute to a temperature which is at least C. below the sintering temperature, and quenching the so fired core in air.

References Cited by the Examiner I UNITED STATES PATENTS 2,882,234 4/ 1959 Gorter et a1 252-625 3,038,860 6/1962 Vinal et a1 252-62.5 3,093,588 .6/1963 Brown 252-62.5 3,160,576 12/1964 Eckert et al. 252-625 3,226,328 12/1965 Esveldt et a1 252-625 TOBIAS E. LEVOW, Primary Examiner.

R. D. EDMONDS, Assistant Examiner. 

